A liquid crystal display device typically comprises a pair of flat panels defining a volume in which a quantity of liquid crystal material is sealably contained. These liquid crystal materials may comprise dichroic dyes in a guest/host system, or twisted nematic materials. The flat panels generally include transparent electrode material on their inner surfaces. One panel may be covered completely by a single transparent ground plane electrode while the other panel is configured with an array of transparent electrodes, referred to as "pixel" (picture element) electrodes. Thus the typical cell in a liquid crystal display includes liquid crystal material situated between a pixel electrode and a ground electrode, forming a capacitor-like structure. Orientation of the liquid crystal material is affected by voltage aplied across the electrodes on either side of the liquid crystal material. A voltage applied to the pixel electrode typically effects a change in the optical properties of the liquid crystal material, resulting in display of information on the liquid crystal display (LCD) screen.
The utilization of transparent front and back panels and transparent electrodes permits the visual effects to be produced by transmission of optical radiation through the LCD device. These visual effects may be facilitated by employing a separate light source for the display, such as a fluorescent lamp type device. LCD display screens can be manufactured to produce color images by incorporating color filter mosaics in registration with the pixel electrode array. Some of these structures may employ polarizing filters to either enhance or complete the desired visual effect.
Liquid crystal displays of the transmissive type require, for proper illumination, flat, extended light sources having high, uniform surface brightness. Shallow depth light sources, moreover, enhance the utility of the liquid crystal display by detracting only minimally from its capability for utilization as a narrow depth display device.
One method of providing a light source with high, uniform brightness and shallow depth is to employ an array of tubular, fluorescent lamps in combination with a front surface diffuser. The diffuser acts to illuminate the dark regions between tubes and provide a uniform source of light over its entire flat, extended surface. However, the diffuser also degrades the average light intensity and this reduces the light source efficiency. End effects resulting from the multitude of cathode connections required to drive such an array of fluorescent lamps also reduce the light source efficiency.
A number of approaches may be taken to simultaneously maximize surface brightness and uniformity of brightness where the light source comprises an array of fluorescent lamps. These approaches include various combinations of diffusers, lenses and reflectors. Such system is set forth by J. L. Henkes in U.S. application, Ser. No. 941,002 filed Dec. 12, 1986 and assigned to the instant assignee, now issued as U.S. Patent No. 4,735,495 on Apr. 5, 1988. As described in that application, which is hereby incorporated by reference in its entirety, a high intensity light source (e.g., a discharge lamp) is positioned in an integrating sphere and tapered light pipes are positioned at an opening in the integrating sphere to produce a collimated output light beam which is projected onto a surface of the liquid crystal panel display. A modification of that approach is described in J. L. Henkes U.S. application Ser. No. 116,198 filed Nov. 3, 1987, U.S. Patent 4,765,718, and assigned to the instant assignee. In the latter application, the disclosure of which is hereby incorporated by reference in its entirety, a secondary collimator is positioned between the end of a light pipe collimator and the flat panel liquid crystal display. This modification reduces the offset of the collimated light beam toward the edge of the collimating light pipe.
Other types of flat light sources such as vacuum fluorescent and electroluminescent may also be used. Such sources, however, do not offer the combined brightness and energy conversion efficiency of low pressure discharge fluorescent lamps.
Accordingly, one object of the invention is to provide a flat panel liquid crystal display illuminated by a flat, uniformly bright, electrodeless lamp having adjustable brightness.
Another object of the invention is to provide a uniformly bright, efficient light source in a flat package suitable for use in illuminating liquid crystal displays.
Another object of the invention is to provide a uniformly bright, electrodeless arc discharge light source having a flat light-emitting surface.
Another object of the invention is to provide an electrodeless arc discharge lamp in a flat enclosure, wherein gases within the discharge space are excited by an array of coils located outside the enclosure behind one surface thereof and energized with radio frequency (RF) current.
Another object is to provide a flat, electrodeless arc discharge lamp that is readily dimmable.